The present invention broadly relates to an accumulator which allows for the thermal expansion of a fluid. It particularly relates to such an accumulator which allows for the thermal expansion of a liquid metal in a zero gravity environment.
There is currently a major combined effort being made by certain government agencies to develop a high-power, light-weight nuclear reactor power source (SP-100 reactor) for use on future spacecraft. The program will require the resolution of numerous technological challenges. One of those challenges will be to provide a means for allowing for thermal expansion of the liquid metal reactor coolant.
The SP-100 reactor utilizes liquid lithium as a reactor coolant and heat transport medium. The temperature of the reactor coolant may range from as low as ambient prior to initiation of reactor power to as high as 1000.degree. Kelvin at maximum power. In addition the means providing for thermal expansion must be reliable since it must last for many years.
To accommodate the thermal expansion of a liquid in a terrestrial environment is a relatively simple and straightforward matter. Specifically, one need only provide an enclosed vessel having an opening in a lower portion thereof. Gravity will result in the more dense liquid staying in a bottom portion of the vessel and air or gas trapped therein will be an upper portion of the vessel by virtue of its lesser density. As the liquid expands the gas will be compressed. Conversely, as the liquid cools, the gas will expand forcing the liquid back out into the system from which it was drawn.
Providing a liquid accumulator for use in a zero gravity environment, such as would be encountered in space, is not so easy a task. In the absence of a gravity gradient, the gas and liquid would intermingle in a device such as just described with the result that the combination of gas and liquid could be anywhere in the system. If this were allowed to occur, the gas could accumulate as a large bubble anywhere in the system for example in a liquid-cooled, nuclear reactor core and interfere with the heat transport of the liquid metal coolant. Specifically, it could result in overheating and failure of individual fuel pins within the reactor core. Obviously, therefore, some means must be provided to accommodate the increase in volume of liquid metal coolant which results from thermal expansion and it must be accomplished in a manner to insure that there is not entrainment of gas in the liquid metal coolant.
It has been proposed to use a bellows to provide for such thermal expansion. The liquid would be on one side of the bellows and the gas on the other such that the flexible movement of the bellows would accommodate expansion and contraction of the liquid while still maintaining the two physically separated. A disadvantage of this approach is that the flexing of the bellows could result in a structural failure or crack the bellows which could in turn result in mixing of the gas and liquid. In addition, the temperatures which would be encountered by the bellows from the coolant in a space reactor environment are such that it is doubtful if a suitable bellows material would be available.
It also has been proposed to use a piston with seals to maintain a gas and liquid separate, while movement of the piston would accommodate expansion or contraction of the liquid. An obvious disadvantage of this approach is that any failure of the seal would again result in a mixture of the gas and liquid with the attendant risk of overheating and failure of fuel pins within the reactor core.
Thus, there clearly is need for a reliable accumulator to allow for thermal expanson of a liquid metal coolant which would be suitable for use in a substantially zero gravity environment.